Metal halide arc discharge lamps are frequently employed in commercial usage because of their high luminous efficacy and long life. A typical metal halide arc discharge lamp includes a quartz or fused silica arc tube that is hermetically sealed within a borosilicate glass outer envelope. The arc tube, itself hermetically sealed, has tungsten electrodes sealed into opposite ends and contains a fill material including mercury, metal halide additives and a rare gas to facilitate starting. In some cases, particularly in high wattage lamps, the outer envelope is filled with nitrogen or another inert gas at less than atmospheric pressure. In other cases, particularly in low wattage lamps, the outer envelope is evacuated.
It has been found desirable to provide metal halide arc discharge lamps with a shroud that comprises a generally cylindrical, light-transmissive member, such as quartz, that is able to withstand high operating temperatures. The arc tube and the shroud are coaxially mounted within the lamp envelope with the arc tube located within the shroud. Preferably, the shroud is a tube that is open at both ends. In other cases, the shroud is open on one end and has a domed configuration on the other end. Shrouds for metal halide arc discharge lamps are disclosed in U.S. Pat. No. 4,499,396 issued Feb. 12, 1985 to Fohl et al. and U.S. Pat. No. 4,580,989 issued Apr. 8, 1986 to Fohl et al. See also U.S. Pat. No. 4,281,274 issued Jul. 28, 1981 to Bechard et al.
The shroud has several beneficial effects on lamp operation. In lamps with a gas-filled outer envelope, the shroud reduces convective heat losses from the arc tube and thereby improves the luminous output and the color temperature of the lamp. In lamps with an evacuated outer envelope, the shroud helps to equalize the temperature of the arc tube. In addition, the shroud effectively reduces sodium losses and improves the maintenance of phosphor efficiency in metal halide lamps having a phosphor coating on the inside surface of the outer envelope. Finally, the shroud improves the safety of the lamp by acting as a containment device in the event that the arc tube shatters.
It is known to provide a metal halide arc discharge lamp having an arc tube that includes an ultraviolet starting aid. U.S. Pat. No. 5,323,091 issued Jun. 21, 1994 to Morris illustrates examples of such arc tubes. The arc tubes described in this patent are known in the art as bubble-in-the-press arc tubes and include a starting aid that is formed in a cavity in a press seal portion of the arc tube. An electrode is provided in the cavity that may extend external to the press seal or be a portion of a molybdenum ribbon used in the press seal.
Although prior art bubble-in-the-press arc tube configurations provide generally satisfactory performance, they have been found to have certain disadvantages particularly regarding the mounting thereof within the shroud of the arc discharge lamp. For example, in a typical metal halide arc discharge lamp that includes a bubble-in-the-press arc tube, it is desired to center the arc tube, the shroud and the shroud holder in the lamp outer envelope. However, heretofore, there has been a tendency for the bubble-in-the-press seal of the arc tube to offset the arc tube within the shroud; that is, the arc tube has tended to be mounted off-center relative to the longitudinal axis of the shroud.
Mounting of the shroud has also incurred disadvantages. In the aforementioned U.S. Pat. No. 4,580,989, two techniques for mounting the shroud are disclosed. According to one technique, the shroud is retained by two metal straps that encircle the shroud and are welded to a frame. According to the second mounting technique, the open end of the shroud is provided with a pair of diametrically opposite slots. The slots engage a metal strap used for mounting of the arc tube.
While both prior art configurations provide generally satisfactory performance, they have been found to have certain disadvantages, particularly in a high volume production environment. In the construction that utilizes a pair of straps that encircle the shroud, the shroud tends to shift lengthwise and/or rotate in the straps when the lamp is jarred during shipping or handling. In the construction that utilizes a pair of slots for engaging a lamp mounting strap, the shroud is subject to cracking or breakage in the areas of the slots where the shroud contacts the strap. Furthermore, it is necessary to heat treat the shroud in the region of the slots to reduce the possibility of cracking or breakage. The heat treatment step adds to the production cost. Accordingly, it is desirable to provide a metal halide arc discharge lamp wherein the shroud is securely locked in position, while cracking or breakage of the shroud is avoided.
U.S. Pat. No. 5,252,885 issued Oct. 12, 1993 to Muzeroll et al. illustrates an electric lamp assembly designed to overcome the foregoing concerns. Such electric lamp assembly includes a sealed lamp envelope, a lamp capsule located within the lamp envelope, a cylindrical, light-transmissive shroud surrounding the lamp capsule, and a mounting arrangement for supporting the lamp capsule and the shroud within the lamp envelope. The mounting arrangement includes one or two support rods parallel to the axis of the shroud and first and second mounting clips for retaining the shroud and the lamp capsule. The clips prevent both axial and lateral movement of the shroud. The clips are attached to the support rods, typically by welding. In a preferred embodiment, a single clip at each end of the shroud retains both the shroud and the lamp capsule, and requires only a single weld to the support rod.
While such electric lamp assembly provides generally satisfactory performance, it has been found to have certain disadvantages in a high volume production environment. For example, in the construction of such an electric lamp assembly, the two mounting clips are welded to opposite sides of the metal support rods that are attached to the lamp stem. Processing the mounting arrangement in this manner complicates the weld process and tends to effect an imbalance in the finished product to the extent that the shroud is subjected to undesirable torque. Such imbalance is undesirable at least to the extent that when such an electric lamp assembly is inadvertently dropped there is a tendency for shroud failure due to such torque. Such failure may be in the form of movement, cracking or breakage of the shroud.